Republican Presidential candidate, former Massachusetts Governor Mitt Romney speaks during a campaign event at the Electronic Instrumentation and Technology company June 27, 2012 in Sterling, Virginia. A recent poll released today shows Romney leading U.S. President Barack Obama in the critical swing state of Virginia by a margin of 5 percentage points.

(AP) WASHINGTON - For nearly 15 years, Republican presidential candidate Mitt Romney's financial portfolio has included an offshore company that remained invisible to voters as his political star rose. Based in Bermuda, Sankaty High Yield Asset Investors Ltd. was not listed on any of Romney's state or federal financial reports. The company is among several Romney holdings that have not been fully disclosed, including one that recently posted a $1.9 million earning suggesting he could be wealthier than the nearly $250 million estimated by his campaign.

The omissions were permitted by state and federal authorities overseeing Romney's ethics filings, and he has never been cited for failing to disclose information about his money. But Romney's limited disclosures deprive the public of an accurate depiction of his wealth and a clear understanding of how his assets are handled and taxed, according to experts in private equity, tax and campaign finance law.

Sankaty was transferred to a trust owned by Romney's wife, Ann, one day before he was sworn in as Massachusetts governor in 2003, according to Bermuda records obtained by The Associated Press. The Romneys' ownership of the offshore firm did not appear on any state or federal financial reports during Romney's two presidential campaigns. Only the Romneys' 2010 tax records, released under political pressure earlier this year, confirmed their continuing control of the company.

The mystery surrounding Sankaty reinforces Romney's history of keeping a tight rein on his public dealings, already documented by his use of private email and computer purges as Massachusetts governor and his refusal to disclose his top fundraisers. The Bermuda company had almost no assets, according to Romney's 2010 tax returns. But such partnership stakes could still provide significant income for years to come, said tax experts, who added that the lack of disclosure makes it impossible to know for certain.

"We don't know the big picture," said Victor Fleischer, a University of Colorado law professor and private equity expert who urged corporate tax code reforms during congressional testimony last year. "Most of these disclosure rules are designed for people who have passive ownership of stocks and bonds. But in this case, he continues to own management interests that fluctuate greatly in value long after his time with the company and even the end of his separation agreement. And the public has no clear idea where the money is coming from or when it will end."

Named for a historic Massachusetts coastal lighthouse, Sankaty was part of a cluster of similarly named hedge funds run by Bain Capital, the private equity firm Romney founded and led until 1999. The offshore company was used in Bain's $1 billion takeover of Domino's Pizza and other multimillion-dollar investment deals more than a decade ago.

Romney's campaign declined to answer detailed questions from AP about Sankaty. Romney aides have said in the past that some disclosures were not required because those assets were valued by his financial advisers at less than $1,000 below the minimum threshold under federal rules set by the U.S. Office of Government Ethics. A financial snapshot of Sankaty in Romney's 2010 tax returns showed the holding with almost no value at the time with $10,000 in both assets and liabilities.

"Everything on the filings is reported as required," campaign spokeswoman Andrea Saul said in a brief statement. "If OGE has an issue with any filings, they would let us know." The agency declined to comment.

While Sankaty no longer plays an active role in Bain's current deals, private equity experts said such holdings could provide significant income to Romney under his 10-year separation agreement from Bain, which expired in 2009. Investment funds typically churn "carried interest," profit shares due to the managers of the funds that often range as much as 20 percent of a fund's annual profit known as "the carry." Even after investment funds are exhausted, profit shares and other late earnings from those stakes can continue to stream, arriving as lucrative "tails," tax experts say. In some circumstances, the analysts added, offshore companies like Sankaty could also offer limited tax deferral advantages.

The implications of Romney's Bain profit-sharing became clear last month when his trust reported that one rarely disclosed asset had posted a $1.9 million payout. The income was described as a "true-up" payment, catch-up income that made up for unpaid earnings owed to Romney as part of his Bain separation agreement.

Such sizable earnings are possible "depending on the terms of the agreement," said tax law expert Michael Kosnitzky, an attorney at the New York firm of Boies, Schiller & Flexner. The Romney campaign acknowledged recently that it could not rule out more large future payments.

The use of offshore companies such as Sankaty is allowed under U.S. tax laws. They are typically set up as shell corporations by private equity and hedge funds to route investments from large foreign and institutional investors, such as large pension plans, into corporate takeovers. The money is used to provide equity and buy up debt. In turn, the investors gain U.S. tax advantages by passing their funds through the offshore "blocker" corporations, avoiding a high 35 percent tax on earnings that the Internal Revenue Service describes as "unrelated business income."